Nafion® (DuPont) or the like, which is a copolymer composed of a hydrophobic segment and an ion-conductive segment, has been heretofore known as a polymer electrolyte for a polymer electrolyte fuel cell. Such polymer has been utilized in the form of a membrane prepared via a membrane-forming technique such as casting.
When such polymer electrolyte membrane is used, moisture supply, such as moistening of an electrolyte membrane or an electrode, is necessary to realize efficient operation of the polymer electrolyte fuel cell. Because of such necessity, conventional polymer electrolyte fuel cells were required to be equipped with complicated piping systems, such as a moisture-supplying system using a pump, blower, or the like and a system for regulating the gas temperature or flow rate to allow the diffusion of the generated moisture, a drive system, and a control system. Such requirement not only inhibits a reduction in the sizes and weights of polymer electrolyte fuel cells, which is a primary advantage of fuel cells, but also involves various difficulties such as the necessity of external energy supply upon diffusion of the generated water, in the case of a low-temperature fuel cell.
JP Patent Publication (Unexamined) No. 5-283094 (1993) discloses a polymer electrolyte membrane comprising a mixture of, for example, Nafion® (DuPont) and polyvinyl alcohol having a hydrophilic segment. In such electrolyte-based fuel cell, an air electrode is allowed to couple to a fuel electrode via a water-absorbing or water-retentive material, such as polyvinyl alcohol. The moisture generated on the air electrode side and the moisture that has migrated from the fuel electrode side to the air electrode side in the fuel cell are collected, and the collected moisture is allowed to migrate to the fuel electrode side via the aforementioned water-absorbing or water-retentive material by the force of surface tension, thereby supplying moisture that is necessary for the fuel electrode. This constitution permits the fuel cells to work without complicated moisture control, eliminates the need for a system for moistening and for water supply, and realizes a reduction in the sizes of fuel cells.
The Proceedings of the 45th Battery Symposium in Japan, Abstract 676-677, 2004 discloses an electrolyte membrane that is produced by mixing aqueous solutions of polyvinyl alcohol (PVA), of 2-acrylamido-2-methylpropane sulfonic acid (AMPS), and of polyethylene glycol (PEG) at a given mixing ratio, agitating the resulting mixture, and spreading the mixture on a teflon sheet or the like for casting. This document also discloses a crosslinked membrane, which is produced by allowing this PVA-AMPS-PEG mixture to react with a solution of glutaraldehyde in acetone or the like for a given period of time.